In Variable Fonts, the axis refers to the area where variations of a typeface can be made between styles, called “masters.” For an axis to be functional, it needs:

• two masters minimum at its extremes;
• compatible contours of every related glyph.

There can be a weight axis (e.g., from a Light to a Bold master), a width axis (e.g., from a Condensed to an Extended master), or many more!

The user can choose precisely which variation is needed by navigating through the axis.